(a) Technical Field
The present invention relates to a gasket for reducing stress concentration in a fuel cell stack. More particularly, the present invention relates to a gasket for reducing stress concentration in a fuel cell stack, which prevents damage or deformation of a separator and further prevents a position shift of the gasket.
(b) Background Art
A fuel cell system is an electricity generation system that converts chemical energy of fuel directly into electric energy. The fuel cell system generally comprises a fuel cell stack for generating electricity, a fuel supply system for supplying fuel (hydrogen) to the fuel cell stack, an air supply system for supplying oxygen in air, which is an oxidizing agent required for en electrochemical reaction, to the fuel cell stack, and a heat and water management system for removing reaction heat of the fuel cell stack to the outside of the fuel cell system and controlling the operation temperature of the fuel cell stack.
The fuel cell system having the above configuration generates electricity by the electrochemical reaction of hydrogen as a fuel and oxygen in the air and exhausts heat and water as the reaction by-products.
The fuel cell stack most widely used for a vehicle is a proton exchange membrane fuel cell (PEMFC) having a high output density.
FIG. 1 is a schematic diagram showing the structure of a conventional fuel cell stack structure, in which a 3-layer membrane electrode assembly (MEA) 1 including an electrolyte membrane transporting hydrogen ions and electrode/catalyst layers, in which electrochemical reaction take place, formed on both sides of the electrolyte membrane, a separator 2 formed on both surfaces of the MEA 1 to form a flow field for working gases and a coolant passage and support the fuel cell, and a gas diffusion layer (GDL) 3 disposed between the MEA 1 and the separator 2 to uniformly diffuse the working gases and transmit generated electricity are repeatedly stacked.
Recently, a support film or a carrier sheet 4 is attached on both surfaces of the MEA 1, except for the catalyst layer, and integrally formed with the MEA 1 to protect the MEA 1 and facilitate the handling of the MEA 1.
In this case, it is necessary to maintain the airtightness between the separator 2 and the MEA 1 to prevent hydrogen and air from leaking to the outside and also prevent fluid leakage between a hydrogen manifold, an air manifold, and a coolant manifold provided in the separator 2.
A gasket 8 is provided to perform such a function of maintaining the airtightness. In general, the gasket 8 is formed of fluorine-based or silicon-based polymer having a Shore A hardness of 50 by injection molding and inserted between the separators 2 or between the separator 2 and the MEA 1. Then, the airtightness is maintained by applying pressure using a connection plate 9 and a connection bolt 14.
When the external connection device is provided and the compression pressure (force) is applied after the fuel cells are stacked, the gasket is deformed to maintain the airtightness.
However, if the deformation amount of the gasket is concentrated in a spot region, the stress concentration is applied to the separator 2 and, in case of a graphite or carbon composite separator having a relatively low elasticity, it may be damaged.
Moreover, in case of a metal separator formed by a sheet metal forming process, there is no damage; however, since the flatness of the separator is not maintained, it is difficult to maintain the airtightness and further the gasket may be shifted from a reference position.
Especially, in case of a fuel cell stack which operates at high pressure, the gasket may be slipped out of the outside of the separator. A groove may be formed on the gasket to prevent the position shift; however, it is also difficult to prevent the separator from being damaged due to the stress concentration.
Meanwhile, a method of manufacturing an MEA integrated with a gasket by forming a hole around the MEA and injecting a liquid rubber material, a method of manufacturing a separator integrated with a gasket by forming a hole around the separator and injecting a liquid rubber material, and the like have been studied. Moreover, a method of forming a hole on a separate carrier sheet and integrating a gasket with both sides of the carrier sheet has been developed. In case of the metal separator, a method of preventing the gasket from being pushed out by forming a bent end part on the edge of the separator, a method of regulating the deformation amount of the gasket by inserting a spacer into the gasket, and a method of integrating the separator, the gasket and the MEA using metal clip members have been disclosed.
For example, U.S. Pat. No. 7,087,339 discloses a fuel cell membrane electrode assembly with sealing surfaces in which a thin rubber plate having a honey-comb structure is adhered to a carrier sheet having a high hardness and integrated with an MEA. Although the '339 patent has advantages in that it is possible to maintain the interval of the separator constant and reduce the assembly tolerance of the fuel cell stack after compression, it requires a large area of the gasket to maintain the airtightness. In case of a separator for a vehicle, it is necessary to increase the output density by maximizing the reaction area in the separator to facilitate the package in the vehicle. Since a gasket having a large area increases the ratio of non-reactive area in the separator, and thus increases the volume of the fuel cell stack, it is not suitable for the intended use.
U.S. Pat. No. 6,231,053 discloses a gasket for fuel cells including a gasket body composed of a metal or resin sheet and having openings and a sealing section formed by injecting a liquid rubber material, in which the sealing section is bonded to both sides of the gasket body. The liquid rubber gasket is formed integrally with an MEA, the sealing section is formed by injecting a liquid rubber material, and the gasket body has openings through which the sealing section is bonded to both sides of the gasket body, thus preventing the gasket from being pushed out by an internal pressure or compression stress concentration. However, although such a structure can effectively prevent the gasket from being pushed out by the internal pressure, if the airtightness is not maintained, hydrogen may be mixed with oxygen to degrade the catalyst layer, thus causing a dangerous situation.
Japanese Patent Application Publication No. 2004-207074 discloses a fuel cell in which a plurality of sealing members are formed by injection molding and a plurality of bent end parts are formed on the edge of a plurality of metal separators to prevent the gasket from being pushed out. In addition, a plurality of spacers are provided at the bent end parts of the plurality of metal separators to regulate the deformation amount of the sealing members or the metal separators. However, such a structure has a drawback in that, since the spacers are added to the outside of the plurality of sealing members, the area of the separator is increased. Moreover, it is expected that it is difficult to stamp the bend end parts on the metal separators and the overall structure is complicated. Furthermore, in the case where hydrogen, oxygen and coolant manifolds are formed in the metal separators, it is difficult to apply such a structure to the small interval between the manifolds.
U.S. Pat. No. 7,063,911 discloses a gasket for a fuel cell and a method of manufacturing the same, in which a connection hole is formed on a separator to connect a gasket to both sides of the separator. A groove portion is formed on the separator to prevent a position shift of the gasket. According to this method, it is possible to prevent hydrogen from being mixed with oxygen; however, it is impossible to apply the gasket to a metal separator formed by a sheet metal forming process.
Japanese Patent Application Publication No. 2004-241208 discloses a fuel cell in which a liquid rubber material is integrated with metal separators by injection molding and a plurality of metal clip members are alternately inserted therein and fixed. However, in the case where hydrogen, oxygen and coolant manifolds are formed in the metal separators, it is also difficult to apply such a structure to the small interval between the manifolds the same as the '074 publication.
U.S. Patent Application Publication No. 2006/0236535 discloses a method of forming a gasket assembly for a PEM fuel cell assembly in which a carrier sheet is die-cut to have a size and shape matching that of a PEM bipolar plate, a plurality of holes are punched in the carrier sheet, a liquid rubber gasket is formed by injection molding, and an MEA is joined to the carrier sheet by laser welding. In addition, a plastic carrier element having a flange portion is attached to the carrier sheet and a spacer is inserted into the carrier element in the vertical direction to restrict the deformation amount. However, this 535 publication is similar to the '053 patent related to the method of manufacturing an MEA integrated with a gasket assembly and the support element is about the same as the third reference except that the gasket is integrated with the MEA, not with the separator.
The information disclosed in this Background section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.